There has been a long felt need for an economically viable process to convert acetic acid to ethanol, which may be used outright or subsequently converted to one or more derivative products such as ethylene. Ethylene is an important commodity feedstock because it can be converted to polyethylene, vinyl acetate, ethyl acetate, or any of a wide variety of other chemical products such as monomer and polymer products. Fluctuating natural gas and crude oil prices contribute to variances in the cost of ethylene conventionally produced therefrom. Thus, the need for alternative sources of ethylene is especially evident when oil prices rise.
Catalytic processes for reducing carboxylic acids, e.g., acetic acid, and other carbonyl group containing compounds have been widely studied. The literature is replete with various combinations of catalysts, supports, and operating conditions for such processes. For example, the reduction of various carboxylic acids over metal oxides is reviewed by T. Yokoyama et al., in which the authors summarize some of the developmental efforts for hydrogenation catalysts for various carboxylic acids. (Yokoyama, T.; Setoyama, T. “Fine chemicals through heterogeneous catalysis. Carboxylic acids and derivatives,” 2001, 370-379, which is incorporated herein by reference in its entirety). It has been difficult, however, to achieve the desired hydrogenation products with conventional catalysts and supports. In some cases, for example, conversion of acetic acid to the desired hydrogenation product is low, while in other cases, selectivity of the desired product, based on the amount of converted acetic acid, is undesirably low.
As one potential solution to this problem, multi-metallic, e.g., bimetallic, catalysts have been suggested for catalyzing the hydrogenation of acetic acid to form products such as ethanol. As one particular example, bimetallic catalysts comprising tin and a second metal have been utilized in ethanol formation. These bimetallic catalysts are typically formed by contacting multiple metal precursors, e.g., a tin precursor and a second metal precursor, with a suitable support in multiple contacting steps and reducing the metal precursors, typically in separate heating steps, in order to impregnate the support with the two metals. Such synthesis processes are undesirable and inefficient, however, due to the required multiple impregnation steps.
A simplified approach for forming bimetallic catalysts involves impregnating a support with multiple metal precursors in a single impregnation step followed by heating to reduce the metal precursors to their corresponding metals. Such single step impregnation processes, however, are not suitable for all metal precursors since some metal precursors are not compatible with one another. This is particularly true of tin precursors, such as tin oxalate, which are generally of low solubility and incompatible with aqueous-based metal precursors.
Thus, the need exists for simplified and efficient processes for forming bimetallic catalysts that include tin.